1. Field of the Invention
The present invention relates to a heating apparatus used in an electrophotographic image forming apparatus.
2. Description of the Related Art
In image forming apparatuses such as electrophotographic copying machines and printers, conventionally, heat roller-type heat fixing apparatuses are widely used for applying heat and pressure to fix, onto a recording material (sheet), an unfixed toner image supported on the recording material by an image forming process such as an electrophotographic process.
From the standpoint of quick start and saving energy, film heating-type heat fixing apparatuses and electromagnetic induction heating-type heat fixing apparatuses, which cause films themselves to generate heat, have also been put to practical use in recent years.
Film heating-type heat fixing apparatuses are proposed, for example, in Japanese Patent Application Laid-Open Nos. S63-313182 and H04-044075.
The film heating-type heat fixing apparatus includes a heater as a heating member, a fixing film as a flexible rotating member which is in contact with the heater and rotates while applying heat, and a pressure roller as a pressurizing member which forms a fixing nip portion with the heater via the fixing film.
In the film heating-type heat fixing apparatus, a recording material supporting thereon an unfixed toner image is introduced between the fixing film and the pressure roller at the fixing nip portion and conveyed together with the fixing film while being sandwiched. Accordingly, the unfixed toner image is fixed on the surface of the recording material by the pressure of the fixing nip portion with the application of heat from the heater via the fixing film. In this heat fixing apparatus, low heat capacity members are used for the heater and the fixing film, and it is sufficient if the heater, which is a heat source, is energized only at the time of executing image formation to generate heat of a predetermined fixing temperature. Accordingly, the heat fixing apparatus has advantages of a short waiting time from power-on of the image forming apparatus to entry into a state in which image formation can be executed and substantially small power consumption at the time of standby.
Japanese Patent Application Laid-Open No. 2003-045615 discloses a metal sleeve for heating, in which a cylindrical metal element tube is used as a base layer and a release layer is provided on the outer surface. In addition, Japanese Patent Application Laid-Open No. H10-10893 discloses a fixing belt, in which a heat resistant elastomer layer is formed on the outer surface of a metal or heat resistant plastic tube and further, a layer of silicone rubber or fluororesin is formed on the outer surface of the heat resistant elastomer layer.
The use of metal, which has higher thermal conductivity than resin, in place of a conventionally used heat resistant resin, such as polyimide, for a base layer of the fixing film increases the thermal conductivity of the fixing film itself, and accordingly, heat from the heater is more efficiently transferred to the recording material. Therefore, it is possible to accommodate the increase in speed of image forming apparatuses. In addition, a fixing film in which metal is used for the base layer has sufficient strength, thus resulting in an increase in endurance and robustness.
Conventionally, there is a problem that fixing unevenness partially occurs, when the toner image passes through the fixing nip portion, because the surface of the fixing film cannot follow the shape of a toner image which is formed by multiply transferring color images. Fixing unevenness appears as gloss unevenness of an image, or leads to transparency unevenness in the case of OHTs (transparent sheets for overhead projectors) and the transparency unevenness appears as an image defect when projected. To deal with this problem, an elastic layer is provided on the base layer of the fixing film so as to render the surface of the fixing film deformable along the toner layer. Therefore, heat is transferred from the fixing film to the toner disposed unevenly on an image in such a manner that the heat is enclosed by the fixing film, thereby achieving uniform fixing performance.
On the other hand, Japanese Patent Application Laid-Open No. H08-016005 discloses an electromagnetic induction heating-type heat fixing apparatus in which, with magnetic fluxes, eddy currents are induced in a film member and Joule heat of the eddy currents heats the fixing film itself. The heat fixing apparatus is able to directly heat the fixing film by using the occurrence of induced currents, and achieves a more highly efficient fixing process compared to a heat roller-type heat fixing apparatus having a halogen lamp as a heat source.
In electromagnetic induction heating-type heat fixing apparatuses, a thin metal is often used for the base layer of the fixing film. Further, in the case where an electromagnetic induction heating-type heating fixing apparatus is used in a color image forming apparatus, a fixing film having an elastic layer provided on the base layer may be used.
Thus, film heat fixing-type and electromagnetic induction heat fixing-type heat fixing apparatuses have been proposed. However, with any type of heat fixing apparatuses, it is necessary to suppress an offset phenomenon caused by toner adhering to the fixing film and again transferred to a recording material.
Accordingly, a release layer including fluororesin, such as polytetrafluoroethylene-perfluoro alkyl vinyl ether copolymer (PFA) and polytetrafluoroethylene (PTFE), may be provided as a surface layer on the base layer or the elastic layer of the fixing film.
In heat fixing apparatuses using the fixing film described above, a lubricant is interposed between the fixing film and the heater or a sliding member, thereby reducing sliding friction between the fixing film and the heater or the sliding member and smoothing the rotational movement of the fixing film.
Because the heat fixing apparatus may be used under high temperature of 180° C. or more, as the lubricant, a fluorine-based lubricant is adopted which shows excellent stability under severe conditions such as high-temperature environments. The basic constituents of a lubricant are a base oil, a thickener, and an additive, and the fluorine-based lubricant is formed of perfluoropolyether (PFPE) as the base oil, a PTFE homopolymer or copolymer as the thickener, and an added material, such as a small amount of a rust inhibitor, as the additive.
PFPE has any of chemical structures represented by Formulae 1 to 4 of FIG. 5, and is broadly categorized into a straight-chain type having a straight-chain structure and a side-chain type having a side-chain structure with a trifluoromethyl group (—CF3) in its side chain.
The kinetic viscosity of straight-chain type PFPE is less dependent on temperature than that of side-chain type PFPE, as illustrated in FIG. 6. That is, straight-chain type PFPE has a lower viscosity than side-chain type PFPE in a low-temperature environment, and has a higher viscosity in a high-temperature environment. For a heat fixing apparatus, it is desired to reduce drive torque required for start-up from a cold condition in a low-temperature environment, and it is therefore preferred that the lubricant have a lower viscosity in a low-temperature environment, which also facilitates the rotation of the fixing film. In addition, in the case where the heat fixing apparatus is used under high temperature such as at the time of continuous printing, it is desired to prevent the lubricant from draining from edges of the fixing film and being depleted from a sliding friction part. Accordingly, it is preferred that the lubricant have a higher viscosity in a high-temperature environment so that run-off of the lubricant can be prevented. Therefore, straight-chain type PFPE is used as a lubricant for conventional heat fixing apparatuses.
However, even when a lubricant formed of straight-chain type PFPE is used, a fair amount of lubricant held between the fixing film and the heater or the sliding member is drained from the edges of the fixing film and comes around to the surface of the fixing film when the rotational movement of the fixing film is continued for a long period of time.
As a result, the fluororesin, such as PFA, used for the surface layer of the fixing film and the lubricant having come around thereto react with each other, which sometimes causes fissures and cracks in the surface layer of the fixing film. These fissures and cracks appear as horizontal lines across the image, thus sometimes resulting in a decrease in image quality.
As for this mechanism, first, PFPE included in the lubricant as a base oil penetrates into the fluororesin, such as PFA, forming a surface layer of the fixing film, which causes swelling of the fluororesin. With this swelling, the distance between polymers in the fluororesin increases, which results in a decrease in strength of the fluororesin itself. Then, fissures and cracks are considered to be caused in the surface layer by application of mechanical stress, such as rotation, to the fixing film.
For such fissures and cracks in the surface layer of the fixing film, Japanese Patent Application Laid-Open No. 2006-126576 proposes to include polytetrafluoroethylene-perfluoroethoxy ethylene copolymer in the surface release layer of a pressurizing member. In addition, Japanese Patent Application Laid-Open No. 2009-25612 proposes that, in a fixing roller having a resin tube covering the elastic layer as the surface layer, the degree of crystallization of the resin tube is 50% or less.
However, according to examinations conducted by the inventors of the present invention, the inventions disclosed in Japanese Patent Application Laid-Open Nos. S63-313182 and H4-44075 reduce flexibility in material selection or sometimes cause unevenness of film thickness in the surface layer.